Try as we might to fight or deny it, old age will impact our running and athletic ability. By age 35, the rates of decline are so predictably linear that this calculator by a Yale economics professor maps out just how slowly you can expect to run the same distance as the years run by.

The calculator, by Ray Fair, PhD, can predict your performance for both short and long-distance running and swimming events, as well as in chess, oddly. Pick your event and input your best finish time and the age when you got your best time. What follows is a table that lists your estimated running times between the age of 35 and 100. According to Fair, you can maintain your top performance up to age 35, then it gets a little sloppier thanks to Father Time.

Fair, who has quite a few sub-4 hour marathon times under his belt, told The New York Times that his calculator has helped runners cope with the inevitable decline of performance and better manage their expectations of their future running times. Of course, these numbers are just estimates based on the factors he published in a paper found in Experimental Aging Research, but they’re close to the models in scientific studies on athlete performance and aging.

An analysis in the Journal of exercise science and fitness suggests that performance declines approximately 1% every year due to a lowered ability to clear out lactate, declining VO2 max (your maximum ability to use oxygen for aerobic work basically), muscle and strength losses which could impact running speed, and even subtler things, like having a smaller appetite and greater life responsibilities that get in the way of training.

Regardless, it still makes sense to keep running whenever and if possible. And if you are passionate about running at your best, all this just means is that you have to adjust your expectations, nutrition, and recovery to the realities of your aging body. There are guides on how to adjust your training so that you can still perform your best at any age.